1. Field of the Invention
This invention relates to an image contour enhancing apparatus for use for signal processing in, for example, a video camera.
2. Description of the Related Art
In an imaging apparatus, such as, a video camera, a horizontal contour enhancing signal component and a vertical contour enhancing signal component are derived from a video input signal, and these signal components are added to the video input signal so as to enhance the image contour thereby enhancing the sharpness of the edge part of the image.
FIG. 7 shows the structure of such a prior art image contour enhancing apparatus. Referring to FIG. 7, in response to a video signal J supplied as an input to the apparatus, a horizontal contour enhancing signal generator circuit 701 generates an output signal for enhancing the contour of the image in the horizontal direction, and a vertical contour enhancing signal generator circuit 702 generates an output signal for enhancing the contour of the image in the vertical direction. The output signal from the horizontal contour enhancing signal generator circuit 701 and that from the vertical contour enhancing signal generator circuit 702 are added in an adder circuit 703. A delay circuit 705 compensates the delay time of the video input signal J supplied to the apparatus. The output signal of the adder circuit 703 and that of the delay circuit 705 are added or composed in another adder circuit 704, and the resultant contour enhanced video signal is outputted from the adder circuit 704.
The operation of the prior art image contour enhancing apparatus will now be described by reference to FIGS. 2 and 8. FIG. 2 schematically shows an image input to the image contour enhancing apparatus. The symbol (J) in FIG. 8 shows the waveform of the video signal J when the image is scanned over periods t0, t1, t2 and t3 in FIG. 2, and the symbols (K), (L), (M) and (N) in FIG. 8 show the signal waveforms appearing from the circuits 701, 702, 703 and 704 respectively in FIG. 7 in response to the supply of the video signal J as the input to the apparatus.
Referring to FIG. 7, in response to the supply of the input signal J to the apparatus, the horizontal contour enhancing signal generator circuit 701 generates the output signal K for enhancing the contour in the horizontal direction, while the vertical contour enhancing signal generator circuit 702 generates the output signal L for enhancing the contour in the vertical direction. The horizontal contour enhancing signal K and the vertical contour enhancing signal L are added or composed in the adder circuit 703 to provide the image contour enhancing signal M, and finally the resultant contour enhanced video signal N appears from the adder circuit 704.
In the prior art apparatus based on the principle described above, high frequency components of a video input signal are extracted and then superposed on the video signal thereby improving the sharpness of the edge part of the image.
However, in the prior art apparatus described above, any especial processing was not made for the image area where both the horizontal and vertical contour enhancing signal components are generated in overlapping relation.
Thus, in the area designated by the numeral 201 in FIG. 2, where the contour enhancing signal components in both the horizontal and vertical directions are generated in the overlapping relation, the value of horizontal contour enhancement (.DELTA.H in FIG. 8) and the value of vertical contour enhancement (.DELTA.V in FIG. 8) are directly added to provide the total value of contour enhancement given by .DELTA.H+.DELTA.V as indicated by the numeral 801 in FIG. 8. As a result, the prior art apparatus has had such a problem that, when the image contour enhancing signal having the coefficient of contour enhancement given by such a value is added to the video input signal, the contour of the image tends to be excessively enhanced to the extent that the viewer of the reproduced image will feel that he is viewing the reproduced image whose edge part is very unnaturally glaring from the visual aspect.